Rabbi Yosei notes that days are longer in the summer and shorter in the winter. (That's true except for on the equator, where day and night are even all year round, and at the poles, where day and night are each half the year.) It's also true that wherever you are on the globe, sunrise and sunset will be at exactly due east and west at the moment of the equinox.

Here's how that works: you can imagine the path of the sun following the edge of a plane that cuts through the dome of the sky. The plane makes an angle with the horizon corresponding to your latitude, e.g., 31.8° in Jerusalem. At the equinox, the plane hits the horizon at exactly due east and due west, so that's where you see sunrise and sunset.

The rest of the year, the plane of the sun hits the horizon elsewhere. In the summer it hits north of east-west line, and in the winter it hits south of the east-west line. The plane moves farthest north at the summer solstice, and farthest south at the winter solstice. The angle bewteen sun and horizon at noon varies between 23.5° above and 23.5° below the noon equinox over the course of the year, reaching those extremes at the solstices. The reason you sunburn faster in the summer is the more direct angle of sunlight when the sun is higher in the sky.

While the ancient sages recognized these patterns in the solar cycle, they didn't understand that they were caused by the tilt of Earth's axis from its orbit, which were finally explained in the 16th and 17th centuries. You might even get the impression from Rav Mesharsheya's berayta, as well as the phrase ribbua olam found throughout the perek, that the Earth is a flat square, not a globe.

Moving on in Eruvin 56a...

Following the topic of equinoxes and solstices, the Gemara here also has the calculations of Shemuel. He considers the length of a year to be exactly 365.25 days, like the Julian calendar. It's clear that he considers that number to be exact, because he assumes that the spring equinox will always fall exactly on a quarter of a day.

The year is in fact closer to 365.242 days, so the Julian calendar gains a bit less than a day each century. That's why the West eventually swapped the Julian calendar for the Gregorian calendar, which skips a leap year three times very four centuries to be much more accurate. The Jewish calendar also relies on the more accurate calculation of Rav Adda bar Ahavah for the length of a year, which is 365 days, 5 hours, 55 minutes, 997 halakim and 48 rega'im.

But the blessing of the sun, birkat ha-hammah, said every 28 years, assumes Shemuel's calculation. As Abaye defines it on Berakhot 59b:

Most other functions of the calendar use the calculation of Adda bar Ahavah. The reason for the exception, I guess, is that without Shemuel's nice round numbers, having the sun return to the exact same spot at an exact time every few decades would be impossible.

Shemul's calculated date of the solstice advances ahead of the Gregorian calendar by about a day per century. This century and last century, birkat ha-hammah is every 28 years on April 8, most recently in 2009 and next in 2037, more than two weeks after the actual equinox around March 20.

Because of this discrepancy, some aharonim express concern that birkat ha-hammah is a berakhah le-vattalah, as reported by Wikipedia (where else do you think I'm getting my facts from?). One report claims that Rav Tzadok ha-Kohen of Lublin rejoiced that it was cloudy on the day of birkat ha-hammah, so that he wouldn't have to worry about this question.

The Pahad Yitzhak has a nice solution to the problem (entry חמה), saying that the berakhah is really appropriate at any time, since it is a general blessing of praise:

And note how he says that Shemuel got the pesak over Rav Adda because his opinion makes the cycle feasible, as I said. Although according to the Pahad Yitzhak, Rav Adda's cycle is merely hard to calculate every 28 years. But if the sun is supposed to reach an exact spot on a Wednesday, it just wouldn't happen at all for at least many thousands of years. I guess the Pahad Yitzhak somehow understood Abaye as independent of Shemuel versus Rav Adda.

The passage here in Eruvin ends with Shemuel connecting weather events to the influence of the planets. Not quite scientific, unfortunately.